Protein kinase B, commonly known as Akt, is a central signaling molecule that directs fundamental cellular activities. It functions as a serine/threonine kinase, modifying other proteins by adding phosphate groups to specific serine or threonine amino acids. Akt plays a major role in regulating cellular processes such as survival, growth, proliferation, and metabolism.
The complexity of Akt signaling is amplified by the existence of protein isoforms, which are closely related forms arising from different genes. In mammals, the Akt family consists of three main isoforms: Akt1, Akt2, and Akt3, each encoded by a separate gene. These three related molecules act as a hub in the cell’s signaling network, linking external cues from growth factors and hormones to internal cellular responses.
The Three Distinct Akt Isoforms
Akt1 is the most widely expressed isoform, found in nearly all tissues. Its primary function is linked to cell growth and proliferation, and it is frequently studied for its role in cancer development. Its widespread distribution suggests a general role in maintaining basic cell viability and responding to growth signals.
Akt2 exhibits a more restricted expression pattern, concentrating in tissues that respond strongly to insulin, such as skeletal muscle, fat (adipose tissue), and liver cells. This isoform is strongly associated with metabolic regulation, particularly the uptake of glucose necessary for maintaining blood sugar balance. Its localized function makes it a primary focus in research concerning conditions like type 2 diabetes.
Akt3 is most highly expressed in the brain, testes, and kidneys. Its unique tissue distribution suggests a specialized role in the function and development of these organs. Akt3 is primarily implicated in brain development and influences processes like cell survival and size in the central nervous system. Despite their functional differences, all three isoforms are activated by the same upstream signaling pathway initiated by phosphoinositide 3-kinase (PI3K).
Quantifying Molecular Weight and Size
Protein size is measured in kilodaltons (kDa), a unit of molecular mass. The three Akt isoforms are similar in size, with their experimentally observed molecular weights falling within a narrow range. These sizes are commonly determined using techniques like SDS-PAGE or Western blotting, which separate proteins based on size and charge.
Experimentally, the Akt isoforms typically have an approximate molecular weight between 56 and 62 kDa. Akt1 is often observed around 62 kDa, and Akt3 is cited close to Akt1. Akt2 is slightly smaller, frequently reported at approximately 56 kDa.
These observed molecular weights are consistently greater than the theoretical size calculated from the amino acid sequences alone. For example, the calculated molecular weight for Akt3 is approximately 55.8 kDa, but it is often detected between 59 and 60 kDa on a Western blot. This difference between theoretical and observed size indicates the influence of factors beyond the primary amino acid sequence.
Structural Basis for Size Variation
The subtle size differences among the Akt isoforms stem from minor variations in their primary amino acid sequences and post-translational modifications (PTMs). All three Akt isoforms share a common structure composed of three distinct domains:
Pleckstrin Homology (PH) Domain
Located at the N-terminus, the PH domain is responsible for anchoring the protein to the cell membrane.
Kinase Domain
This central region contains the catalytic machinery that adds phosphate groups to target proteins.
Regulatory Domain
Found at the C-terminus, this domain helps control the protein’s activity.
Slight variations in the lengths and amino acid composition of these domains account for the inherent differences in the theoretical weights of Akt1, Akt2, and Akt3.
A more significant contributor to the observed size variation is Post-Translational Modification (PTMs). These are chemical additions to the protein that occur after it has been translated. The most studied PTM is phosphorylation, the addition of a phosphate group, which adds mass to the protein. Akt requires phosphorylation at two sites—one in the activation loop and one in the C-terminal regulatory domain—to become fully active. The addition of these phosphate groups, along with other PTMs like ubiquitination or acetylation, increases the total mass, causing the fully activated form of Akt to appear larger than its inactive counterpart in laboratory separation.
Functional Impact of Physical Differences
The subtle structural variations among the Akt isoforms directly influence their function and cellular behavior. The unique sequence of the N-terminal PH domain in each isoform affects its binding efficiency to the cell membrane. Since membrane binding is the first step in Akt activation, differences in the PH domain structure dictate how quickly and strongly each isoform responds to cellular signals.
These structural differences also influence substrate specificity—which proteins each isoform prefers to phosphorylate. While all three isoforms phosphorylate many common targets, minor sequence changes, especially within the kinase domain, lead to a preference for certain substrates. This variation explains how Akt2 selectively regulates glucose metabolism in insulin-responsive tissues, while Akt1 focuses more on growth.
These physical distinctions have clear implications for disease, particularly concerning Akt2 and insulin signaling. The specific structure of Akt2 dictates its role in promoting the translocation of the GLUT4 glucose transporter to the cell surface, a process that is essential for glucose uptake in muscle and fat cells. Therefore, structural or regulatory defects unique to Akt2 have a disproportionate effect on metabolic disorders. Understanding these precise physical distinctions helps researchers develop highly specific therapies that target one isoform without broadly affecting the functions of the others.